In the context of the present application, RF signals will be understood to mean analog signals comprising a plurality of frequencies, e.g. frequencies in the frequency band between 45 and 1040 MHz. A large number of different devices for receiving RF signals (radio frequency signals) can be found on the market. Examples for such devices are TV tuners for cable modems, DVD recorders, Set Top Boxes, USB dongles, PC add-on cards, TV's and other RF apparatuses. In such devices, a RF input signal is applied to an input of the device and the device is adapted to sample the RF input signal. In such devices, for example in case of TV tuners, the RF input signal comprises a plurality of frequencies and channels and a required channel selection and frequency conversion is performed in the device in order to provide a user with specific signals corresponding to particular channels.
In certain cases, the RF input signal has to be applied to more than one appliance, for example to a DVD recorder and to a TV apparatus. To this purpose, the RF input signal, which is an antenna signal for example, can be first split and then routed to each of these appliances. However, this is inconvenient for users and requires an additional RF splitter device. Therefore, the devices for receiving RF signals, such as DVD recorders, are usually equipped with a so called loop-through output that is adapted such that the RF input signal is fed out again via a loop-through output to which a further appliance can be connected. In this case, the RF input signal, e.g. the antenna signal, is first routed to the first appliance, for example the DVD recorder, and the second appliance, e.g. a TV apparatus or the corresponding RF receiver, can then be connected to the loop-through RF output of the first appliance. The concept of loop-through is well known and extensively used in existing home RF appliances.
In conventional devices of this kind, the RF input signal is provided to an input, and typically buffered with a low-noise amplifier (LNA) forming a part of the analog pre-processing circuitry, and then made available at a loop-through output (LTO) for use by another device for receiving RF signals. In the conventional devices, stringent requirements are placed upon the avoidance of deterioration of signal quality by the analog pre-processing circuits. The analog pre-processing circuits therefore are relatively complex and require relatively large power dissipation which is disadvantageous in view of aspects like heat generation energy consumption, reliability and cost.
Recently, concepts for further improving such devices for receiving RF signals have been developed. A RF receiver that could be used in a DVD recorder is shown in FIG. 1 as an example for a device for receiving a RF signal 100. The device shown in FIG. 1 is implemented as a direct-sampling multi-channel RF tuner. As can be seen in FIG. 1, a RF input signal 2 is applied to an input 3. The RF input signal 2 is buffered with a low-noise amplifier (LNA) 4 and is made available at the loop-through output (LTO) 5 for use by another device for receiving RF signals. From the LNA 4 the RF input signal 2 is passed to a variable gain adjustment circuit (VGA) 6, an anti-aliasing circuit 7, and an analog-digital converter (ADC) 8 converting the RF input signal 2 to a digital signal 9. The analog-digital converter 8 is clocked by a clock signal fclock. The digital signal 9 is provided to a digital signal-processing unit (DSP) 10 digitally processing the signal.
In the device of FIG. 1, the RF signal spectrum applied at the input 3 is sampled in its entirety and the channel selection and frequency conversion is done in the digital domain, i.e. in the digital signal-processing unit (DSP) 10. A main advantage of such a concept is that a plurality of channels contained in the RF input signal can be received concurrently.
In the case that this concept of multi-channel direct sampling is used, new possibilities for the implementation of the analog pre-processing circuits are available. Digital signal processing techniques implemented in the digital signal-processing unit 10 can be used to compensate linear and non-linear distortions resulting from the analog pre-processing circuits. Since this possibility of compensating the linear and non-linear distortions in the digital signal processing unit 10 is provided, a simplified analog pre-processing circuitry 11 can be used resulting in increased non-linear distortions. In particular, a less complex low-noise amplifier 4 at the input can be used. Thus, the complexity and the power dissipation of the analog pre-processing circuitry 11 can be significantly reduced by such distortion compensation. In the upper part of FIG. 2, a) represents the RF input signal 2 at the input terminal 3; b) represents the signal after the analog pre-processing circuitry 11 comprising the LNA 4, the VGA 6, and the anti-aliasing circuit 7; and c) represents the signal after the digital signal processing unit 10. As indicated in b), the signal after the analog pre-processing circuitry 11 contains non-linear distortions schematically indicated by the two smaller arrows which are not present in the RF input signal schematically indicated in a). The digital signal processing in the digital signal processing unit 10 is then used to compensate these non-linear distortions introduced by the analog pre-processing circuitry 11, as shown in c).
However, if the digital signal processing in the digital signal processing unit 10 is used to enable simplification of the analog pre-processing circuitry 11 including the low-noise amplifier 4 acting as a buffer, the problem arises that distortion generated in this buffer is not compensated in the loop-through RF signal provided at the loop-through output 5.